A charged particle detector is an indispensable part of a charged particle (ion or electron beam) instruments, for example, a scanning electron microscope (SEM). In a SEM, an electron beam emanated from an electron source is focused into a fine probe over a specimen surface and scanned by a deflection unit in a raster fashion; and signal electrons released from the specimen, including secondary electrons and back scattered electrons, are collected by charged particle detectors and the signal intensity is converted into the gray level of an image pixel corresponding to the location of the electron probe on the specimen surface. Scanning of the electron probe will then form a gray level mapping thus producing an image of the specimen surface.
There common used detectors in SEM are scintillator-photomultiplier tube (PMT) combination type (e.g. an Everhart-Thornley detector), semiconductor type, and microchannel plate type. The scintillator-PMT type, due to their high gain and low noise properties in respect to the semiconductor type and microchannel plate type, is more frequently used in a high resolution and low beam current SEM. Traditionally, this type of detector is consisting of a light guide, a scintillator disc is attached to the front face of the light guide, and the rare end of the light guide is coupled to a photomultiplier tube. Secondary electrons and backscattered electrons emit from sample surface impinged on scintillator disc and, in response, generate light signals. The light guide collects the light signal and directs it to PMT. In a conventional design, the electrons to light signal conversion efficiency and light signal collection are low. In order to compose an image with enough brightness and contrast, a large magnification PMT or magnifying circuit is needed, which will introduce a larger electric noise into the image. Since the electron to light conversion efficiency is depending on the chosen scintillator material, thereby, it is expected to improve efficiency of the light collection before the PMT.
FIG. 1 illustrates a typical SEM system with a prior art electron detection device that is positioned above objective lens. The SEM configure with an electron source 101, a gun lens 102, and objective lens 103. Primary electron beam 112 generate from electron source 101 moving along the optical axis 113 through the center hole of a detection device strike sample 104 surface. There are several positions to set the detection device to intercept signal electrons (backscattered and secondary electrons) emanating from the sample 104 surface. A detection device set as detector 111 is called through the lens detector for collecting electrons with higher kinetic energy such as backscattered electrons 105. A detection device set as detector 200 is called side detector for collecting electrons with lower kinetic energy such as secondary electrons 106. The side detector 200 comprises a metal grid 204, a scintillator disc 201, a light guide 202, and a PMT 203. The signal from PMT is then being processed to become an image of sample surface.
FIG. 2A is a schematic illustration of a cross-section along the center axis of a conventional charged particle detection device 200. The metal grid 204 usually contains 100V to 500V positive potential respect to the sample 104 surface. Secondary electrons 106 emanate from the specimen 204 surface are attracted by the potential applied on the metal grid 204 to the detection device. After passing the metal grid, the secondary electrons accelerate to the scintillator disc 201 due to a 5 kV to 15 kV positive potential that applied to the scintillator disc 201. The high speed secondary electrons bombard the scintillator disc 201 and generate photons (light). The photons generated in the scintillator disc 201 propagate through the light guide 202 and reach the PMT 203 then become a current signal. In this design, the center of the scintillator disc, the center of the PMT, and the center of the light guide are all aligned by the center axis 220.
A charged particle detective device as FIG. 2A can be used to detect both positive and negative charged particles. In order to detect negative charged particles, the metal grid 204, and the scintillator disc 201 are applied a positive potential to lure and to accelerate negative charged particles such as electrons. On the other hand, for detective positive charged particles, the metal grid 204, and the scintillator disc 201 are applied a negative potential to lure and to accelerate positive charged particles such as sputtered gallium ions and secondary ions in Focused Ion Beam (FIB) system.
The light receiving efficiency of a detection device is defined in the present invention that the intensity of light received at the end of a light guide per unit energy input to a scintillator disc. According to the definition, a conventional detection device with a design as FIG. 2A has a light receiving efficiency around 25% when the light guide is made of BK7 glass with 120 mm in length and the scintillator disc is made of CEYAG.
There were many scientists put their efforts on collecting charge particles and signal electrons in different environment, but seldom discussion on the light ray receiving efficiency of the detection device. U.S. Pat. No. 4,900,932 by Schafer et al., disclosed a cathodoluminescence detector which includes an elliptical hollow mirror and a tube having a reflecting inner surface for conducting the light emitted by a specimen under investigation in a scanning electron microscope. The invention is collect and conducts the light emitted by specimen instead of from a scintillator. U.S. Pat. No. 6,069,356 by Todokoro, described the secondary electrons collecting mechanism in the scanning electron microscope application. U.S. Pat. No. 6,943,352 by Hayn, disclosed an apparatus to detecting charged particles in a gaseous environment during imaging in a scanning electron microscope. U.S. Pat. No. 7,417,235 by Schon et al., disclose an apparatus can detecting secondary ions as well as secondary electrons and tertiary electrons. U.S. Pat. No. 7,462,839 by Ganuck et al., disclosed a detector for scanning electron microscopes which can be used under different pressure conditions in the specimen chamber. The detector is designed for detection of both electrons and light.
The present invention propose a new high light receiving efficiency detection device to detect charged particles. The high receiving efficiency is achieved through altering the shape of light guide and the shape of scintillator disc.